The Placebo Effect

There is a need to differentiate between a perceived placebo effect and a true placebo effect.¹

Perceived placebo effect

This is seen in the placebo arm of a clinical trial, e.g. new antihypertensive drugs may drop the average blood pressure by 5mmHg, but this may not be true placebo effect as other factors may confound the situation. Such factors include:

• The natural course of the disease - blood pressure may normally reduce over time thus exaggerating the placebo effect.
• Regression towards the mean - biological variables often fluctuate.² When first measured they are probably approaching their maximum and so further measurements are likely to show a reduction.
• Increased skill of the investigator - this may alter measurements up or down as more measurements are made.
• Variable factors within the same patient - white coat hypertension may reduce as patients become accustomed to having their blood pressure measured which would increase the apparent placebo effect.
• Non-apparent simultaneous changes - on entering the trial, the patient may change their behaviour, on purpose or otherwise, in such as way as to confound the outcome, e.g. eat less salt, take more exercise.

True placebo effect

This can only be studied if an untreated group is included along with active and placebo treated groups. However, these are relatively uncommon but it has been discovered that:

• Placebo treatment is more effective in relieving pain compared with no treatment. To achieve this, patients need to be conscious (placebo was given to sleeping patients and no difference noted).
• Objective clinical parameters can be changed by placebo treatment, e.g. oedema and an increase in C-reactive protein level following oral surgery.
• Physical placebos, e.g. sham acupuncture are more powerful than simple oral placebos.³
• Topical placebo is also more effective than oral placebo, e.g. in primary varicose veins.

There is no standard degree of placebo effect (approx. 30% is often given) and its extent depends upon numerous factors including:

• The demeanour of the person offering treatment.
• The patient's attitude to health and their feelings about the treatment and person offering it.
• The suggestibility of the patient.
• The form of treatment, e.g. whether it has worked before, how expensive it is, its invasiveness and the reasons given as to how it works.
• Placebo effects on pain are generally greater than on other symptoms.
• Placebo effects are not always useful and may produce adverse effects (nocebo effects).